Heads-up displays allow a user to view a scene while relevant information is overlaid on the scene, so that the user looking through the heads-up display simultaneously sees both the scene and the relevant information. For example, a pilot looking through a heads-up display while landing an airplane simultaneously sees the airport ahead (the scene) through the heads-up display while the heads-up display projects information such as speed, heading and altitude (the relevant information) that the pilot needs to land the plane.
In some uses of a heads-up display it can be useful know what part of the scene the user is viewing. One way to accomplish this is through eye-tracking technology, but existing eye-tracking technologies have some disadvantages. Among other things, existing eye tracking technologies use an optical path separate from the optical path used for the display, making the heads-up display more bulky and complex and less streamlined.